Significance

Methane from global rice cultivation currently accounts for one-half of all crop-related greenhouse gas emissions. Several international organizations are advocating reductions in methane emissions from rice by promoting intermittent flooding without accounting for the possibility of large emissions of nitrous oxide (N2O), a long-lived greenhouse gas. Our experimental results suggest that the Indian subcontinent’s N2O emissions from intermittently flooded rice fields could be 30–45 times higher than reported under continuous flooding. Net climate impacts of rice cultivation could be reduced by up to 90% through comanagement of water, nitrogen, and carbon. To do this effectively will require a careful ongoing global assessment of N2O emissions from rice, or we will risk ignoring a very large source of climate impact.

Abstract

Global rice cultivation is estimated to account for 2.5% of current anthropogenic warming because of emissions of methane (CH4), a short-lived greenhouse gas. This estimate assumes a widespread prevalence of continuous flooding of most rice fields and hence does not include emissions of nitrous oxide (N2O), a long-lived greenhouse gas. Based on the belief that minimizing CH4 from rice cultivation is always climate beneficial, current mitigation policies promote increased use of intermittent flooding. However, results from five intermittently flooded rice farms across three agroecological regions in India indicate that N2O emissions per hectare can be three times higher (33 kg-N2O⋅ha−1⋅season−1) than the maximum previously reported. Correlations between N2O emissions and management parameters suggest that N2O emissions from rice across the Indian subcontinent might be 30–45 times higher under intensified use of intermittent flooding than under continuous flooding. Our data further indicate that comanagement of water with inorganic nitrogen and/or organic matter inputs can decrease climate impacts caused by greenhouse gas emissions up to 90% and nitrogen management might not be central to N2O reduction. An understanding of climate benefits/drawbacks over time of different flooding regimes because of differences in N2O and CH4 emissions can help select the most climate-friendly water management regimes for a given area. Region-specific studies of rice farming practices that map flooding regimes and measure effects of multiple comanaged variables on N2O and CH4 emissions are necessary to determine and minimize the climate impacts of rice cultivation over both the short term and long term.

Edited by Paul G. Falkowski, Rutgers, The State University of New Jersey, New Brunswick, NJ, and approved August 3, 2018 (received for review June 11, 2018)

Significance

Methane from global rice cultivation currently accounts for one-half of all crop-related greenhouse gas emissions. Several international organizations are advocating reductions in methane emissions from rice by promoting intermittent flooding without accounting for the possibility of large emissions of nitrous oxide (N2O), a long-lived greenhouse gas. Our experimental results suggest that the Indian subcontinent’s N2O emissions from intermittently flooded rice fields could be 30–45 times higher than reported under continuous flooding. Net climate impacts of rice cultivation could be reduced by up to 90% through comanagement of water, nitrogen, and carbon. To do this effectively will require a careful ongoing global assessment of N2O emissions from rice, or we will risk ignoring a very large source of climate impact.

Abstract

Global rice cultivation is estimated to account for 2.5% of current anthropogenic warming because of emissions of methane (CH4), a short-lived greenhouse gas. This estimate assumes a widespread prevalence of continuous flooding of most rice fields and hence does not include emissions of nitrous oxide (N2O), a long-lived greenhouse gas. Based on the belief that minimizing CH4 from rice cultivation is always climate beneficial, current mitigation policies promote increased use of intermittent flooding. However, results from five intermittently flooded rice farms across three agroecological regions in India indicate that N2O emissions per hectare can be three times higher (33 kg-N2O⋅ha−1⋅season−1) than the maximum previously reported. Correlations between N2O emissions and management parameters suggest that N2O emissions from rice across the Indian subcontinent might be 30–45 times higher under intensified use of intermittent flooding than under continuous flooding. Our data further indicate that comanagement of water with inorganic nitrogen and/or organic matter inputs can decrease climate impacts caused by greenhouse gas emissions up to 90% and nitrogen management might not be central to N2O reduction. An understanding of climate benefits/drawbacks over time of different flooding regimes because of differences in N2O and CH4 emissions can help select the most climate-friendly water management regimes for a given area. Region-specific studies of rice farming practices that map flooding regimes and measure effects of multiple comanaged variables on N2O and CH4 emissions are necessary to determine and minimize the climate impacts of rice cultivation over both the short term and long term.

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